linux-kernel - Re: O_DIRECT to md raid 6 is slow

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Message-ID: <5033A06B.30508@xs4all.net>
Date:	Tue, 21 Aug 2012 16:51:23 +0200
From:	Miquel van Smoorenburg <mikevs@...all.net>
To:	stan@...dwarefreak.com
CC:	David Brown <david.brown@...bynett.no>,
	Michael Tokarev <mjt@....msk.ru>,
	Linux RAID <linux-raid@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: O_DIRECT to md raid 6 is slow

On 08/20/2012 01:34 AM, Stan Hoeppner wrote:
> I'm glad you jumped in David.  You made a critical statement of fact
> below which clears some things up.  If you had stated it early on,
> before Miquel stole the thread and moved it to LKML proper, it would
> have short circuited a lot of this discussion.  Which is:

I'm sorry about that, that's because of the software that I use to 
follow most mailinglist. I didn't notice that the discussion was cc'ed 
to both lkml and l-r. I should fix that.

> Thus my original statement was correct, or at least half correct[1], as
> it pertained to md/RAID6.  Then Miquel switched the discussion to
> md/RAID5 and stated I was all wet.  I wasn't, and neither was Dave
> Chinner.  I was simply unaware of this md/RAID5 single block write RMW
> shortcut

Well, all I tried to say is that a small write of, say, 4K, to a 
raid5/raid6 array does not need to re-write the whole stripe (i.e. 
chunksize * nr_disks) but just 4K * nr_disks, or the RMW variant of that.

Mike.
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